Tripwire Security Systems, Inc.  
   Tripwire, file integrity assessment tool for UNIX.
   http://www.tripwiresecurity.com/

Deception Toolkit  
   Make your system seem to have network security bugs, and log attack attempts.
   http://all.net/contents/dtk.html

FakeBo  
   Fakes trojan servers (Back Orifice, NetBus) and logs every attempt from client.
   http://cvs.linux.hr/fakebo/

Samhain  
   A tool for file integrity checking that optionally can be used as a client/server application for centralized monitoring of networked hosts.
   http://la-samhna.de/samhain/

TCP Wrappers and other security tools  
   This is the main FTP site for security tools and papers from Wietse Venema.
   ftp://ftp.porcupine.org/pub/security/index.html

Flawfinder  
   Examines source code looking for security weaknesses.
   http://www.dwheeler.com/flawfinder/

COPS: UNIX Security Checker Software  
   Tool to check for trojans, CERT advisories, and system file permission correctness.
   http://dan.yosemite.ca.us/cops/

Cyber Security and Information Infrastructure Protection  
   Practical measures for critical infrastructure protection, intrusion prevention and threat assessment.
   http://www.citi.umich.edu/u/provos/cybersecurity/

Libsafe  
   Free software (GNU license) from Bell Labs for protecting internet servers from buffer overflow attacks, one of the most commonly exploited security vulnerabilities.
   http://www.research.avayalabs.com/project/libsafe/

Guarded Memory Move (GMM)  
   The GMM library can be used to prevent and study buffer overflows exploits
   http://www.xmailserver.org/gmm.html